Thousands of sub-micron devices (e.g., logic gates) in ultra large scale integrated (ULSI) circuits are in data communication with each other via interconnects. As device geometry continues to scale down for ULSI circuits, there is a growing demand for device interconnects with smaller pitch and higher conductivity. Copper interconnects are now being used to meet this demand and to provide other benefits such as high conductivity and high electromigration resistance.
ULSI circuits are formed on semiconductor wafers using well known tools and processes. FIGS. 1A-1E illustrate cross-sectional views of a portion of an exemplary semiconductor wafer 10 during the manufacture of integrated circuits thereon. More particularly, FIG. 1A shows a portion a dielectric 12 formed on wafer 10. FIG. 1A also shows trenches 14 formed within dielectric 12. Although not shown, the bottom and sides of trenches 14 are lined by a thin barrier of tantalum or tantalum nitride to prevent copper electromigration after trenches 14 are filled with copper to create copper interconnects.
FIG. 1B shows wafer 10 after a layer of copper 16 has been deposited on dielectric 12. Copper fills trenches 14 to eventually form copper interconnects. After formation of copper layer 16, wafer 10 in FIG. 1B undergoes chemical-mechanical polishing (CMP) using a conventional CMP tool. FIG. 1C shows that most of the copper layer 16 deposited on dielectric 12 is removed by chemical-mechanical polishing. The remaining copper forms copper interconnects 20.
Chemical-mechanical polishing recesses surfaces 22 of copper interconnects 20 below surface 24 of dielectric 12 as seen in FIG. 1C. The recessing effect of chemical-mechanical polishing can vary across the surfaces 22 of copper interconnects 20 as shown in FIG. 1C. More particularly, interconnects 14 are shown recessed below the surface of dielectric layer 12 more so towards the middle of the copper interconnects than at the ends of the interconnects, thus creating a dish like surface 22 on copper interconnects 14.
In the past, dielectric materials, such as silicon nitride, silicon dioxide, silicon carbide, or silicon carbon nitride, were used to cap copper interconnects such as those shown within FIG. 1C. The copper interconnects were capped with a dielectric material for several reasons; the dielectric cap acts as an etch-stopping layer, a copper diffusion barrier, or a copper corrosion barrier. In all cases, the added dielectric layer needs to be relatively thick to be effective. Unfortunately, a thick dielectric cap formed over the copper interconnects creates a structure with unacceptably high capacitance and is also prone to electromigration (EM) and stress migration (SM) failures due to interfacial issues between the dielectric and the copper. To address the high capacitance and reliability problems, dielectric caps have been replaced with more electrically conductive caps formed out of refractory metals or alloys. Cobalt is a conductive material commonly used to create caps over copper interconnects.
Three main techniques are used to cap copper interconnects with cobalt: (1) selective metal chemical vapor deposition (CVD); (2) selective electroless plating, and; (3) blanket deposition (CVD or plating) followed by CMP planarization. Unfortunately, these prior art techniques can reduce reliability of resulting integrated circuits. To illustrate, FIGS. 1D and 1E show operational aspects of the last technique, blanket deposition followed by CMP. In FIG. 1D, a cobalt layer 26 is formed on the exposed surfaces of dielectric 12 and copper interconnects 20 of the wafer 10 shown in FIG. 1C using conventional CVD. Thereafter, wafer 10 undergoes a second mechanical-chemical polishing. FIG. 1E shows wafer 10 after excess cobalt is removed via the chemical-mechanical polishing, resulting in cobalt caps 30 on copper interconnects 20. It is noted that the capacitance produced by the structures shown within FIG. 1E is less than the capacitance produced by copper interconnects with a dielectric cap formed thereon and should also have improved reliability.
Chemical-mechanical polishing recesses copper interconnects 20 below the surface of dielectric 12 as shown in FIG. 1C. Proper recessing is needed to create effective, more reliable cobalt caps (e.g., caps 30 in FIG. 1E) on copper interconnects 20. The recesses resulting from chemical-mechanical polishing is highly dependent on the geometry of the copper interconnects. More particularly, the recessing of copper interconnects below the dielectric surface is more dominant in large patterns of copper interconnects than in smaller or more density-packed patterns. In other words, a sufficiently deep recess is created in the copper when the width of the copper interconnect is substantially large and/or when the distance between copper interconnects is substantially large.
The depth of the recess can vary by pattern density. For copper interconnects with small widths or for copper interconnects closely positioned next to each other, the copper interconnect recesses created during chemical-mechanical polishing may not be sufficient. To illustrate, FIGS. 2A and 2B show cross-sectional views of a wafer 30 during manufacture of integrated circuits thereon. Wafer 30 shown in FIG. 2A includes dielectric 32, copper interconnects 34 and recesses 36, wherein the recesses 36 were created during chemical-mechanical polishing to remove excess copper from the surface of dielectric layer 32. Presuming the width of the copper interconnects 34 is smaller than the widths of copper interconnects 20 shown within FIGS. 1C-1E and/or presuming the distances between copper interconnects 34 is smaller than the distances of copper interconnects 20, the recesses 36 created in copper interconnects 34 by chemical-mechanical polishing are substantially smaller than the recesses shown within FIG. 1C.
The small recesses shown in FIG. 2A can create problems. To illustrate, after chemical-mechanical polishing, a layer of cobalt is deposited on wafer 30. Wafer 30 is then subjected to chemical-mechanical polishing. FIG. 2B shows wafer 30 after cobalt deposition and subsequent chemical-mechanical polishing. As can be seen, cobalt caps 40 are formed on copper interconnects 34. Cobalt caps 40, however, do not completely cover copper interconnects 34. As a result, portions of copper interconnects 34 may experience subsequent corrosion or other adverse effects.
FIGS. 3A and 3B illustrate another adverse effect that can occur with narrow or closely-spaced copper interconnects. More particularly, FIGS. 3A and 3B show cross-sectional views of a portion of an exemplary wafer 50 during manufacture of integrated circuits thereon. FIG. 3A shows wafer 50 after chemical-mechanical polishing to remove excess copper deposited on the surface of dielectric 52. Sometimes, as can be seen in FIG. 3A, chemical-mechanical polishing removes a portion of the dielectric between the copper interconnects 54 to create a recessed surface 56 on dielectric 52. Removal of dielectric between copper interconnects often occurs when the copper interconnects are densely packed. The copper interconnects 54 in FIG. 3A are more densely packed than the copper interconnects 20 shown in FIG. 1C, or in other words the distances between copper interconnects 54, as shown in FIG. 3B, is less than the distances between copper interconnects 20. FIG. 3B shows the result of depositing a cobalt layer on wafer 50 and subsequently removing excess cobalt during a second chemical-mechanical polishing. In FIG. 3B, it can be seen that the remaining cobalt creates a conductive path between and thus shorts copper interconnections 54.
Selective metal CVD or selective electroless plating may also create problems when capping copper interconnects. These problems are illustrated with reference to FIGS. 4A-4C, which show cross-sectional views of a portion of an exemplary wafer 60 during manufacture of integrated circuits thereon. Wafer 60 includes copper interconnects 64 retained in trenches of dielectric 62 after chemical-mechanical polishing to remove excess copper from the dielectric surface. During selective metal CVD or selective electroless plating, cobalt will deposit preferentially on the copper interconnect surface. FIG. 4B shows a result of selective CVD or electroless plating deposition of cobalt to form cobalt caps 70 over copper interconnects 64. As can be seen in FIG. 4B, cobalt caps 70 when formed may extend over and cover portions of dielectric 62. The overhang or the portions of cobalt caps 70 extending over the dielectric 62 could lead to electrical leakage problems. Additionally, some cobalt may also deposit on defects or contamination on the dielectric surface. FIG. 4C shows deposits 72 of cobalt that form on defects (not shown) in the surface of dielectric 62. Cobalt deposits 72 may also lead to electrical leakage problems.